The increase in both the performance and portability of electronic devices creates challenges in that the underlying integrated circuits (ICs) require greater amounts of power to provide a greater degree of functionality and accuracy, while the physical dimensions of the devices are progressively reduced to improve portability.
Voltage regulators are a particular type of circuit that is necessary for the functionality of portable electronic devices. They are used to convert a supply voltage from one value to a different value, typically from a high voltage to a lower voltage. A voltage regulator may be used to convert an input supply voltage to an operating voltage that is applicable to, or more efficient for, a particular circuit. In some cases, voltage regulators may be used to provide current to a load at a stable voltage, despite a varying supply voltage, such as from a battery. In other cases, both of these functions may be important. For example, the battery voltage in a typical mobile phone is nominally 3.6 volts, but varies considerably as the battery discharges. Some of the integrated circuits within a mobile phone are designed to operate at voltages as low as 1.1 volts, to minimize power consumption. Voltage regulators are generally required to provide a constant output voltage regardless of whether the input voltage or the load current varies. Regulation is achieved by sensing the output voltage and adjusting the energy supplied to the output circuit in response to departures from the target voltage. In a linear regulator this adjustment takes place continuously, but in a switching regulator energy is supplied in discrete pulses which may vary in frequency and duration.
Switch mode conversion operates by transferring pulses of energy from a voltage source to storage elements such as inductors or capacitors, and redistributing that energy in such a way that the required voltage or current is delivered to the load with minimal losses. This is accomplished with control and switching circuitry that regulates the rate at which energy is transferred to the output. The duty cycle, that is, the ratio of on/off time, of the switching circuitry controls the amount of energy available at the load.
In a modern battery powered device, the output current required to supply a microprocessor IC might vary from around one ampere when the device is performing a processor-intensive task, such as processing high-quality graphics, to around 200 microamperes when the device is in a sleep mode. In sleep modes, the device's processor may be doing nothing more than waking up occasionally to perform housekeeping activities or respond to user input. Since the battery life of portable devices is critically important to customers, improving that battery life is important to device designers. The efficiency of switching regulators is generally best at moderate to high output currents (see, for example, U.S. Pat. No. 5,994,885), and falls off at lower output currents due to current in the control and switching circuitry, which at low output currents consumes a higher percentage of the total output power. To this end, the typical prior art solution adopted is to combine a switching regulator, for use when the required load current is high, with a linear regulator, for use when the load current is low (see, for example, U.S. Pat. No. 7,880,456 and U.S. Pat. No. 7,990,119).
A linear regulator includes an active pass device, such as a field-effect transistor or bipolar transistor. Effectively, the pass device is controlled, using feedback from the regulator output, to act as a variable resistance, so that the output voltage is maintained at a desired level, regardless of variation in the load current or in the input supply voltage. Those skilled in the art are aware that linear regulators become less efficient as the difference between input and output voltage increases, because of the resistive voltage drop across the pass device (see Aivaka White Paper WP1: “Linear or LDO Regulators & Step-Down Switching Regulators”).
Both types of regulator require a finite time to respond to changes in the load current, and to this end one or more capacitors are connected across the regulated output to store a reservoir of charge which can supply the short term current requirements of the load. This means that the output voltage will depart from the nominal voltage when the supply current or the load current changes. Product design ensures that under normal operation the voltage ripple is sufficiently small that functionality is not degraded.
In a switching regulator one or more inductive elements may additionally be used to store energy for supply to the output. When the output current is high, the inductive elements maintain the output current in between consecutive switching pulses. If the stored energy in the inductive elements does not drop to zero between pulses this is termed “continuous mode”, and typically includes the region of maximum conversion efficiency. When the switching regulator operates at low currents, the on-time or the frequency of energy pulses may be reduced to match the rate of energy supply to the output demand. If the current becomes low enough the energy in the inductive components is allowed to drop to zero between switching pulses (“discontinuous mode”). During the off period between energy pulses the output voltage is maintained by the external capacitance. The control circuit normally remains active so that it can respond to increased current demand when it occurs but constant energy losses in the control circuit mean that conversion efficiency in the discontinuous mode falls off as the output current is reduced.
The invention described herein provides techniques for products with sleep modes which, by making use of the available knowledge on predictable load variations and voltage ripple tolerance across different states, reduce the energy requirement of the control circuitry of a switching regulator. This extends the efficiency of a switching regulator further into the low current region, providing improved efficiency in lower power states when compared to a linear regulator, such that the need for a linear regulator nay be removed.